1. Field of the Invention
This invention relates to an optical disc allowing information to be recorded and/or reproduced optically thereon. Also, the present invention relates to a method and apparatus for fabricating a master disc to be used for forming the optical disc.
2. Description of the Prior Art
Generally, the optical disc is formed from a master disc by copying. The master disc is provided by a photolithography. In detail, a photo resist film is formed on a substrate. The photo resist film is exposed to a laser beam switched by an exposure pulse. The exposed photo resist film is developed by a developer to produce the master disc with pre-pits thereon. The pre-pits formed on the master disc have a difference length according to a period in which the laser beam is irradiated on the photo resist film. The irradiating period of the laser beam varies with the width of the exposure pulse. Consequently, the length of each pre-pit is determined by the width of the exposure pulse, as shown in FIG. 2. For example, first and second pre-pits of short length are formed by the laser beam being irradiated on the photo resist film during the exposure periods of 3 T and 4 T, while fourth pre-pit of longer length is defined by which the laser beam is irradiated on the photo resist film in the exposure period of 10 T. The pre-pits formed as above is inverted and transcribed to provide a stamper to be installed into a disc molder. The disc molder receives a molten substrate material toward the stamper and allows the molten substrate material toward the stamper to be cooled, thereby forming a disc substrate. On the disc substrate, pre-pits having a shape equal to them on the master disc are formed. Finally, a reflective film and a protective film for preventing the deterioration of the reflective film are sequentially disposed on the disc substrate to produce the optical disc. Such a process of fabricating the optical disc is adapted to a read only optical disc such as CD-ROM and DVD-ROM, an once writable disc such as CD-R, DVD-R and so on, and a rewritable disc such as CD-RW, DVD-RW and so on.
Also, the recording area of a writable optical disc has concentric tracks, one or two spiral track. The tracks are formed in the structure of a land and groove allowing a trace control to be accurately performed. The land and groove tracks are formed by the process described as above, as well as the pre-pits.
Further, the optical disc is gradually higher in the recording density to record a great amount of information such as a moving picture. Actually, the CD has a track pitch of 1.6 xcexcm and a minimum pit length of 0.69 xcexcm, whereas the DVD large relative to the CD in the recording capacity has a track pitch of 0.74 xcexcm and a minimum pit length of 045 xcexcm. This represents that the recording density of the DVD is very higher than that of the CD. In order to enlarge the recording density of the optical disc, these are researched and developed methods using a light source of short wavelength such as a blue laser and increasing the aperture of an objective lens. Since the track pitch and pit length of the optical disc is gradually small, as the recording density is higher, the fabrication of the master disc requires a high accurate.
However, the width of the exposure pulse, i.e., the irradiating period of the laser beam varies the width of the pre-pit as well as the length of the pre-pit. In real, the pre-pit 7 of a long length created by a exposure pulse having a width of 10 T is large relative to the pre-pits 1 and 3 of short length formed by the exposure pulse each having width of 3 T and 4 T, in the width. This results from that the power of the laser beam to be irradiated on the photo resist film is relative to the width of the exposure pulse. Actually, the laser beam is light when the width of the exposure pulse is small, while the laser beam becomes powerful as the width of the exposure pulse is enlarged. By the laser beam, the width of the pre-pit is gradually extended until the length of the pre-pit is arrived at 5 T, and is maintained constantly when the length of the pre-pit is at least 5 T. The variation of the width of the pre-pit is enlarged more and more as the recording density becomes higher. Due to this, the laser beam responding to the exposure pulse can not form the pre-pit having a desired width and length, as the recording density is higher. Also, since the pre-pit is smallest, as the recording density is higher, a reproducing signal capable of reproduction can not detected. As a result, errors are generated in the signal reproduced from the optical disc having a high recording density. The reproducing error makes the pre-pit have at least critical length and the optical disc have not a high recording density.
Accordingly, it is an object of the present invention to provide an optical disc having a high recording density.
Another object of the present invention provides a method and apparatus of fabricating a master disc which are used for the fabrication of an optical disc having high recording density.
In order to achieve these and other objects of the invention, an optical disc according to an aspect of the present invention is provided with a pit having a width which is larger relative to a length.
A method of fabricating a master disc according to another aspect of the present invention comprises steps of: providing a substrate; forming a photo resist film on the substrate; and exposing repeatedly the photo resist film at least twice to form one track on the substrate.
A method of fabricating a master disc according to another aspect of the present invention comprises steps of: providing a substrate; forming a photo resist film on the substrate; and exposing simultaneously the photo resist film to at least two lights to form one track on the substrate.
A method of fabricating an optical disc according to another aspect of the present invention comprises steps of: providing a master disc produced by the master disc fabricating method as described above; transcribing a master disc to provide a stamper; and molding a disc substrate using the stamper.
An apparatus for fabricating a master disc according to still another aspect of the present invention comprises exposing means for performing repeatedly an exposure of a photo resist film at least twice to form one track.
An apparatus for fabricating a master disc according to still another aspect of the present invention comprises: light means for generating laser lights; processing means for forming a channel bit stream from an exposure data; modulating means for switching the laser lights in response with the channel bit stream; information generating means for obtaining a rotating information in accordance with the rotation of the substrate; and light deflective means for moving the axis of the laser lights toward any one of inner and outer circumference on the substrate responding to the rotating information.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.